The present invention relates to an integrally formed screw such as a woodscrew. Wood screws are used in many different applications. Some applications require a wood screw combined with a soft, polymeric washer. In such applications, it is often required that when the screw and the polymeric washer is in place, the screw head be as unobtrusive as possible in order to avoid damage to the screw head or other problems when working on the surface from which the screw heads project, e.g. when shoveling snow from the roof.
The known screws have a flat underside which engages the polymeric washer. The flat underside often causes a non-uniform deformation of the soft washer whereby the sealing or locking performance of the washer is lost or reduced.
My U.S. Pat. No. 5,919,020describes a wood screw in which the specific pressure exerted by the screw head on a workpiece is relatively low, the fastening force high and the top surface of the head relatively unobstructive. This is achieved by combining the shape of a round head screw with an integrally formed radial extension resembling a washer having a flat underside. While this type of screw enjoys a considerable commercial success, in use with a polymeric, e.g. rubber washer the disadvantages mentioned in the preceding paragraph could be encountered.
It is an object of the present invention to provide a screw, particularly, but not exclusively, a wood screw, which would avoid or at least reduce the problem of undesired lack of uniformity of compression of a soft washer while securing the unobtrusive state in a fully applied condition and providing strength sufficient for use with power screwdrivers.
In broad terms, the invention provides an integrally formed screw having a circular head defining a peripheral rim, and an integrally formed stem which includes a thread formed in at least a part thereof. The stem has a root end at the head of the screw, and a free end portion. The head includes an underside from which the stem projects. The top surface portion of the head, facing away from the stem, includes a screwdriver engaging recess. An annular underside of the head is disposed about the root end of the stem. In accordance with the invention, the underside of the head defines a shallowly concave annular portion co-axial with the stem and decreasing in depth in the radial direction toward the circumference of the head.